Tunable lasers are in high demand in optical communication systems because of their versatility. They are widely deployed in core networks but so far have been too expensive for use in access networks, i.e. in data transmission devices located at the customers' premises. Designs which are currently in use typically rely on sophisticated optoelectronic laser chips. An alternative is the extended cavity laser (ECL) which uses a low-cost gain chip combined with a mechanically tuned thin-film filter and a ball lens for collimation. The mechanically tunable laser design commonly offers three parameters with which the laser wavelength can be adjusted: the filter position (coarse-tuning of the laser gain profile), the laser current (fine-tuning of the cavity modes), and the laser temperature (fine-adjustment of the phase).
Using the laser temperature to fine-adjust the phase requires a costly Peltier cooler/heater and has the additional disadvantage of a slow response speed. As an alternative a cavity mirror may be moved by using a piezo-electric element which requires moving parts and therefore is expensive and not robust enough. Furthermore a semiconductor phase tuning section may be used, however, this needs optical pumping. Due to coupling losses between chips the gain and phase control section should be integrated on a single chip which requires sophisticated lithography which again is costly.